What is this serial number to?

According to my Cat SNID book it is listed as an R4 & were first produced in 1935. If it has a diesel probably someone repowered it with a diesel engine.Look on the left side of the diesel block towards the back below the head & see if it has a serial number for the engine & if it,s not the same as the tractor then I,m guessing someone repowered it with the diesel.

Hi Bender, According to my data, I can confirm that 6G1553 is an RD4, manufactured early 1942 (Serial numbers for 1942 ran from 6G1516 to 6G2942). I understand that from 6G1 to 6G876 the tractor was designated a "Thirty" as cast into the radiator tank top, from 6G877 it changed to RD4.
The SP in your serial number means "Special Parts", I believe that means that special (non standard) gear ratios were installed in that tractor. I have a D2 ser No. 5J2720SP and it was fitted with different ratios for 3rd and 4th gear according to the parts book, I believe for agricultural purposes. It could well be the same for the R4.
As SJ states, many R4's were re-engined with a diesel for economic reasons (probably when the gas engine needed a major overhaul)...well, they were here in UK where gas (petrol) has always been more expensive than in US.
Assuming your engine is a "D4" engine, and has an Eiseman Magneto Model RC-2H, the points gap should be 20 thou, according to my D4 servicemans reference book.

Hope this helps !

John, these books drive you up a tree with SNs don,t they. This Cat SNID book I have says the RD4 was a 4G SN & came out in 1936 so that,s all I have on it. I remmber seeing a couple 4Gs come into the shop to work on back probably in the late 50s & early 60s.

thank you for your info. The serial number on the engine is 5T19WSP. The mag on the pony motor is an American Bosch MJK 4/23600-47. I wondered if the engine may not have been a replacement.

A 5T should be a mid 40's D4 engine and should be a D4400 if on the side of the block if the Sn on the rear is a 6G one or the other has been swapped out
Jason

A 5T should be a mid 40's D4 engine and should be a D4400 if on the side of the block if the Sn on the rear is a 6G one or the other has been swapped out
Jason

Hi, Just realised I made a typo in my earlier post the 6G ser number is an R4 (not RD4, oops !). 5T19WSP looks like a D4 tractor ser No. the W denotes wide gauge as all D4 had the same prefix, unlike e.g. D2 which were 3J narrow and 5J wide gauge. Don't know why the D4 was numbered differently.